An organic EL element has a structure in which a layer containing an organic fluorescent compound is arranged between a cathode and an anode, and an electron and a positive hole were injected into the light emission layer and recombined to form an exciton. The element emits light, utilizing light (fluorescent light or phosphorescent light) generated by deactivation of the exciton, and the element can emit light by applying a relatively low voltage of from several to several decade volts. The element has a wide viewing angle and a high visuality since the element is of self light emission type. Further, the element is a complete solid element, and is noted from the viewpoint of space saving and portability. Therefore, study for practical application of the element has been made.
Practical application of a multicolor light emission apparatus, which forms an image by light emission from plural organic EL elements emitting red light (R), plural organic EL elements emitting green light (G), and plural organic EL elements emitting blue light (B) arranged, has been attempted.
A conventional multicolor light emission apparatus, employing plural organic EL elements, is difficult to be manufactured, and further, a multicolor light emission apparatus with high precision is not manufactured stably.
It is known that in the light emission layer there is are a hole transporting type light emission layer with high hole transportability, an electron transporting type light emission layer with high electron transportability, and a bipolar type light emission layer with high hole transportability and high electron transportability. This type is determined due to the kinds of the dopant and the host constituting the light emission layer, a combination of the dopant and the host constituting the light emission layer, or the content ratio of the dopant to the host in the light emission layer. For example, when the electroluminescent element comprises an electron transporting type light emission layer, the requirement must be satisfied that the excited energy of material constituting a hole transporting layer adjacent to the electron transporting type light emission layer is higher than that of the dopant which is contained in the electron transporting type light emission layer, and contributes to light emission. Otherwise, a high light emission efficiency cannot be obtained which results from excited energy transfer from the dopant to the material of the hole transporting layer, or light emission from the hole transporting layer often occurs, resulting in fluctuation of chromaticity. This also applies to the relationship between the hole transporting type light emission layer and the electron transporting layer adjacent thereto or the relationship between the bipolar light emission layer and the hole transporting layer adjacent thereto or the electron transporting layer adjacent thereto. The dopant content not only determines a hole or electron transporting ability, but also has an influence on emission lifetime or emission efficiency, and therefore, cannot be determined only by the transporting ability.
The electroluminescent element is designed considering the above. However, as the hole or electron transporting ability is determined by kinds of the dopant or host or their content, it is extremely difficult to employ the same hole transporting material, the same host or the same electron transporting material in the BGR elements, and therefore, the degree of freedom of design or manufacture of the BGR elements is low. For example, when a full color display is manufactured employing the three color emission elements, an organic EL element emitting red light, an organic EL element emitting green light, and an organic EL element emitting blue light, it is necessary that in each color emission element, the layer constitution, the content of compounds used in the layer or the layer thickness be designed, separately. This is a troublesome process in which the layers in each color emission element are formed, separately, employing a shadow mask. It is expected that when each color emission element is manufactured employing the same compounds of known compounds or so as to have a layer with the same thickness in order to minimize load on the manufacture, there occurs the problem in that color fluctuation is produced or emission efficiency is lowered.
In employing the same compounds in each color emission element, there is disclosed in Japanese Patent O.P.I. Publication No. 2000-82582 a multicolor light emission apparatus reducing the total number of compounds used in which the same compound is employed as the host material and electron transporting material in the red light emission layer, which can reduce burden in the manufacture.
However, this multicolor light emission apparatus emits red light employing aluminum tris (8-quinolinolate) (Alq3) in the light emission layer of the organic EL element, and when voltage is applied to emit light, the difference between chromaticity on the high voltage side and that on the low voltage side tends to be greater. This is because light is emitted not only from the dopant but also from the aluminum tris (8-quinolinolate) on the high voltage side. Particularly in red color, the chromaticity difference has a great influence on chromaticity (see FIG. 13).